A pressure filter of the above-described type is known from EP 0 479 077 A1. This pressure filter works with a continuous cleaning of the filter surface and is especially destined for the separation of solids from suspensions. To the fundamental construction of such a pressure filter a plurality of pressure-tight filter elements separating the fluid compartment from the filtrate compartment and forming the filter surface and the backwashing means connected to the reject drain and having a backwashing head sealing a part of the filter surface with respect to the fluid compartment belong. The backwashing head and the filter surface are movable with respect to one another. The fluid to be filtered is supplied to the pressure filter with a pressure P.sub.1 and acts on the filter elements with this pressure P.sub.1 in the fluid compartment. The filtrate is drained from the filtrate compartement with a lower pressure P.sub.2. In the reject drain and backwashing compartment restricted by the backwashing head a pressure P.sub.3 is adjustable which is always smaller than the pressure P.sub.2 in the filtrate compartment. A filtration occurs on account of the pressure difference between the pressure P.sub.1 of the fluid compartment and the pressure P.sub.2 of the filtrate compartment, wherein solids are deposited on the filter surface and form a cover layer. This cover layer is continuously cleaned by backwashing, i.e. a conversion of the flow direction through the filter elements. The backwashing occurs at that portion of the filter surface which is sealed by the backwashing head with respect to the fluid compartment with a pressure difference between the pressure P.sub.2 in the filtrate compartment and the pressure P.sub.3 in the backwashing compartment. The filtration process at the remaining filter surface of the pressure filter is not influenced by the backwashing.
According to the known pressure filter the backwashing compartment has at least one suction chamber open to the filter surface in a slot-like manner. This suction chamber is formed as elongated chamber extending from a central drive shaft for the backwashing head radially outwardly. Upon rotation of the backwashing head the slot-like aperture sweeps over the filter surface as a radial beam emanating from the center of the pressure filter. Now, it became evident that difficulties result to assure at each radial point of the slat-like or plate-like backwashing head the same pressure against the filter surface. So, on account of the central support of the backwashing head, the pressure is higher in the central range than in the radial outer range. Especially, it became evident in practice that the radial outer end portions of the slat-like or plate-like head tend to lift off from the filter surface so that the correct realization of the backwashing progress can no more be guaranteed.